In conventional electronically controlled fuel injecting device for internal combustion engines, the amount of fuel supplied to the engine would be increasingly corrected in consideration of deterioration of the fuel volatility during cold-engine operation or in consideration of a transiently increased amount of fuel adhered onto the inner wall surface of the intake passage (the increased flow rate of fuel flowing along the inner wall surface) during accelerating operation.
The requirement for increasing correction of fuel supply is different depending on the degree of the fuel volatility, that is, the degree to which the fuel is heavy or light. In case of heavier gravity fuel of a lower volatility, there is a great requirement for increasing correction of fuel supply. In general, it is difficult to perfectly maintain the degree of the fuel volatility, namely the degree to which the fuel is heavy or light, at a constant value. At least in case of heavy-gravity fuel used for the engine, in the conventional electronically controlled fuel injecting device, a characteristic necessary for the fuel-supply increasing correction suitable for heavy-gravity fuel would be preset, so as to satisfy the requirement for increasing correction of fuel supply.
However, under a condition in which the fuel-supply increasing-correction characteristic suitable for heavy-gravity fuel is held constant, if the characteristic for heavy-gravity fuel is applied to light-gravity fuel in case of which there is almost no necessity of increasing correction of fuel supply, there is a tendency of excessive increasing correction of fuel supply, thereby resulting in an excessively rich air-fuel ratio. As a result, unburned hydrocarbons (HC) contained in the exhaust gases tend to increase. Thus, it is desired that the level of increasing correction of fuel supply is suitably varied depending on the volatility of fuel supplied to the engine. For the reasons set forth above, it is necessary to detect a volatility (heavier-gravity or lighter-gravity) of fuel supplied to the engine.
One such technology, in which the level of increasing correction of fuel supply is corrected to a proper level based on the fuel volatility, has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 5-195840.
In the above-mentioned Publication, torque fluctuations in an internal combustion engine, resulting from surge, is monitored and the amount of increasing-correction of fuel is gradually decreased depending on the monitored torque fluctuations (surge torque fluctuations), and thus the increasing-correction amount is controlled so that the surge torque does not exceed an acceptable level. Therefore, the amount of increasing-correction is decreasingly corrected, as the fuel volatility becomes high. As a consequence, the increasing-correction amount of fuel can be provided in a manner so as to conform to the fuel volatility.
However, in case of the prior-art apparatus described in the above-noted Publication, owing to a rapid decrease in the amount of increasing-correction, there is a possibility that the amount of increasing-correction is reduced excessively to such an extent that it exceeds an optimal level for increasing-correction, and thus great surge torque fluctuations, having a bad influence on driveability, occurs. For the reasons set out above, it is difficult to increase a speed of decreasing-correction necessary for the amount of increasing-correction.
Therefore, it takes a long time until an optimal level of the amount of increasing-correction has been reached. The conventional apparatus suffers from the drawback that a period of time for improvement of exhaust-emission control characteristics, based on the optimal amount of increasing-correction, is limited. Thus, it is desired that the correction necessary for adjustment of the increasing-correction amount to its optimal level (in other words, detection of a fuel property) can be achieved for a short period of time.
In the previously-described conventional apparatus, since a fuel-property is detected by varying an air-fuel ratio toward a maximum permissible lean air-fuel mixture, the engine operation will be unstable until the detection of the fuel-property has been finished. Therefore, a judgment for the fuel-property could be made only during idling in which the engine operation is comparatively stable. Furthermore, there is an additional problem that the conventional apparatus tends to be easily affected by disturbance.
It is, therefore, in view of the above disadvantages, an object of the present invention to provide an apparatus which is capable of detecting a property of fuel for a short period of time just after the engine starts, and certainly detecting the fuel property, irrespective of variations in engine operation.